1. Field of the Invention
The present invention relates to a multi-nozzle ink jet recording device, and more specifically to a highly reliable multi-nozzle ink jet recording device capable of automatically detecting defective nozzles and restoring normal printing without performing a test-pattern printing.
2. Related Art
Japanese Patent Publication No. SHO-47-7847 discloses a conventional ink jet recording device formed with a plurality of nozzles aligned in a line in a widthwise direction of a recording sheet. Ink droplets ejected from the nozzles impact and form dots on the recording sheet while the recording sheet is moved in a sheet feed direction perpendicular to the widthwise direction, thereby forming dot images on the recording sheet. The ejected ink droplets are uniform in their size and separated one from the other.
The recording device also includes electrodes that generate a charging electric field and a deflector electric field for respective nozzles. The charging electric field charges the ejected ink droplets based on a recording signal, and the deflector electric field having a uniform magnitude changes a flying direction of the charged ink droplets as needed, thereby controlling the impact positions of the ink droplets with respect to the widthwise directions so as to form the dots on exact target positions.
There has been also proposed a nozzle array where a plurality of nozzles are formed in an arrayed manner, which improves recording speed. However, increase in the number of nozzles sacrifices the reliability of the device.
Air bubbles and any foreign substances existing within a nozzle will result in ink droplets ejected at an angle and also in a splash where unintended minute ink droplets are generated. In worse case, no ejection is performed.
Moreover, when the ejection direction is angled or when the splash is caused in this manner, ink droplets may impact and cling on the electrodes. Especially, the splashed minute ink droplets have a low flying speed and a greater deflection amount because of their small diameter, so a large number of minute ink droplets cling on the electrodes. Because the ink has been charged by the charging electric field, the ink clinging on the electrodes increases an electric current conducting through the electrodes. Therefore, a nozzle corresponding to the electrodes with increased electric current can be easily detected defective.
However, the above method for detecting defective nozzles is not useful in a recording device including common electrodes used common to a plurality of nozzles. Specifically, when there is any change in electric current, it can be known that there is a defective nozzle(s). However, because the amount of change in the electric current due to a single defective nozzle is unknown and fluctuates, it is impossible to detect the number of defective nozzle(s) or to identify the defective nozzle(s). For example, even if defective ejection is detected when droplets are ejected from two nozzles at one time, it cannot detect which one of the two nozzles is defective or whether both of the two nozzles are defective.
It is conceivable to perform a test-pattern printing where an ink droplet is ejected from each one of the nozzles one at a time. Then, it is determined whether or not each nozzle is defective or normal by using a laser beam or a CCD sensor. However, this method is time consuming and wastes ink, and it is impossible to perform such a time-consuming test-pattern printing during actual image forming operations. Moreover, there is hardly a case when an ink droplet is ejected from only a single nozzle during the actual printing, not the test-pattern printing, so it is unrealistic to wait such a single-nozzle printing during the actual printing to detect defectiveness of each nozzle. That is, even when a nozzle becomes defective during printing, there has been no conventional means for restoring the normal printing during the printing, so that there has been no choice but to continue the defective printing with the defective nozzle.
There is a choice to temporarily stop the printing to detect a defective nozzle. However, this method wastes time required for the detection, wastes ink contributed for the detection, and requires disposing the ink contributed for the detection. Accordingly, it is preferable to avoid such an operation as much as possible.
It is therefore an objective of the present invention to overcome the above problems, and also to provide a highly reliable multi-nozzle ink jet recording device capable of automatically detecting defective nozzles and restoring normal printing without performing a test-pattern printing.
In order to overcome the above and other objectives, there is provided an ink jet recording device including a head, electrodes, first detecting means, and identifying means. The head is formed with a plurality of nozzles through which ink droplets are selectively ejected based on ejection data during printing. The electrodes are provided common to the plurality of nozzles. The first detecting means detects whether all of selected nozzles through which ink droplets are ejected are normal or at least one of the selected nozzles is defective. The identifying means automatically identifies a defective nozzle while the head is continuously performing the printing when the first detecting means detects that the ejection is performed defective.
There is also provided a detecting method of detecting a defective nozzle among a plurality of nozzles formed to a head of an ink jet recording device that includes the head and electrodes for generating a deflection electric field common to the plurality of nozzles. The detecting method includes the steps of a) detecting whether all of selected nozzle through which ink droplets are ejected are normal or at least one of the selected nozzles is defective, and b) identifying a defective nozzle among the plurality of nozzles while the head continuously performing the printing when the ejection is detected defective in step a).